Show in the top bar when a newer version is available

At start and after switching server, the game asks the server for the newest
MusicPlayer release and shows "Version X is available" in the top bar when it
is newer than this one.

main/top_bar.gd

@@ -13,6 +13,7 @@ signal tab_selected(tab: StringName)
 
 @onready var sync_button: Button = $SyncButton
 @onready var debug_label: Label = $DebugLabel
+@onready var update_label: Label = $UpdateLabel
 @onready var quiz_tab: Button = $QuizTab
 @onready var statistics_tab: Button = $StatisticsTab
 
@@ -27,6 +28,36 @@ func _ready() -> void:
 	$QRButton.pressed.connect(qr_pressed.emit)
 	quiz_tab.pressed.connect(tab_selected.emit.bind(&"quiz"))
 	statistics_tab.pressed.connect(tab_selected.emit.bind(&"statistics"))
+	check_for_update()
+
+
+## Asks the server for the newest MusicPlayer release and tells the user when it is newer
+## than this one.
+func check_for_update() -> void:
+	MusicServerClient.fetch_latest_client_version(show_latest_version)
+
+
+## Shows that [param latest] is available when it is newer than this version, and hides
+## the message otherwise.
+func show_latest_version(latest: Variant) -> void:
+	var newer: bool = latest is String && is_newer_version(latest, Settings.version)
+	update_label.visible = newer
+	if newer:
+		update_label.text = "Version %s is available" % latest
+		update_label.tooltip_text = "You have %s" % Settings.version
+
+
+## Whether version [param latest] is newer than [param current], comparing numbers such as
+## [code]1.10.0[/code] part by part. A leading "v" is ignored, and missing parts count as 0.
+static func is_newer_version(latest: String, current: String) -> bool:
+	var latest_parts: PackedStringArray = latest.strip_edges().trim_prefix("v").split(".")
+	var current_parts: PackedStringArray = current.strip_edges().trim_prefix("v").split(".")
+	for i: int in maxi(latest_parts.size(), current_parts.size()):
+		var a: int = latest_parts[i].to_int() if i < latest_parts.size() else 0
+		var b: int = current_parts[i].to_int() if i < current_parts.size() else 0
+		if a != b:
+			return a > b
+	return false
 
 
 ## Shows [param tab] as the chosen tab, without emitting [signal tab_selected].